The present invention is directed to an improved method and apparatus for determining the focus and tracking of a light beam from a reflective medium, and, in particular, the invention is directed to a method and apparatus for differential wax-wane focusing and push-pull tracking for a magneto-optical data storage device.
In an optical disk data storage system, light from a laser diode is focused onto the surface of a rotating optical disk. Data is stored on the disk in a track or tracks. The optical disk may have a single spiral track or multiple concentric tracks. Data storage on rotating media requires position sensing information to be included on a part of the data storage surface so that the data storage systems can retrieve information recorded on that surface. Positioning information is typically encoded on the disk surface in a series of pits and grooves. As the disk rotates, the light from the laser diode should be focused in the plane of the disk and on the track centerline. A focusing servo system generates a focus error signal that controls the positioning of the laser diode with respect to the disk surface. A tracking servo system generates a tracking error signal that controls the positioning of the laser diode with respect to the track centerline. The focus error signal and a track error signal control actuators that are coupled to the laser diode to properly focus and align the laser diode with respect to a desired track. The focus and tracking error signals are typically generated by combining the electrical signals from a series of photodetectors that capture the light reflected off the optical disk surface. In addition, a reflectivity data signal which has clock and positioning information, and magneto-optical (MO) data signal which represents the encoded data are generated. Typical magneto-optical systems use up to twelve detector elements to generate the four required signals: the focus error signal (FES), the tracking error signal (TES), the reflectivity data signal (RDS) and the magneto-optical data signal (MODS). Other designs place additional optical elements, such as astigmatic lenses or diffraction gratings, in the light path to generate the four signals.
Therefore, it is desirable that a simpler method and apparatus generate the focus error signal, tracking error signal, the reflectivity data signal and the magneto-optical data signal.
It is therefore a primary object of the invention to provide a less complex apparatus and method for generating the focus error signal.
It is a related objective to provide a less complex apparatus and method for generating the tracking error signal.
It is also an object of the invention to generate the reflectivity data signal and the magneto-optical data signal using the less complex apparatus and method.
The present invention includes an apparatus for determining the focus of a light beam from a reflective medium. The apparatus has a prism and a photodetector. The prism receives the light beam and outputs optical beams. The photodetector has subdetectors. Two adjacent subdetectors form a first pair, the other two adjacent subdetectors form a second pair. The prism projects the optical beams onto the photodetector such that one of the optical beams is projected on at least one subdetector of the first pair of the subdetectors, and another of the optical beams is projected on at least one subdetector of the second pair of the subdetectors. The amount of light projected on the subdetectors indicates the focus of the light beam with respect to the reflective medium.
In another embodiment, one of subdetectors of each pair receives more light than the other subdetector of that pair when in focus.
In yet another embodiment, a disk drive includes the apparatus of the present invention that determines the focus of a light beam from a reflective medium.
In addition, a method for generating a focus error signal is provided. A light beam is received from a reflective medium and first and second light beams are generated from the received beam. The first light beam is projected onto a first pair of subdetectors in a photodetector; and the second light beam is projected onto a second pair of subdetectors in the photodetector. Each subdetector generates a signal in response to the portion of the light beam projected on that subdetector. Two composite signals are determined by adding the signals from diagonally opposite subdetectors in the photodetector. The focus error signal is generated by subtracting one composite signal from the other.
Other features and advantages of the present invention will become apparent to a person of skill in the art who studies the following invention disclosure given with respect to the following disclosure.